Different regulated d.c. voltages have been provided by a power supply comprised of a transformer having its primary winding coupled to a power source of alternating current voltage and each of a plurality of secondary windings coupled to individual rectifying means. The d.c. voltage supplied by a main rectifying means, i.e. the one providing the greatest amount of power, may be regulated by controlling a pulse width modulator coupled to the primary winding, and the d.c. voltage of each of the other rectifying means, which are known as auxiliary rectifying means, is regulated by its own regulator that employs what is known as a post regulation technique. In one such technique a series regulator is used. It is comprised of an impedance through which the load current passes and means for increasing its value if the d.c. voltage goes above a desired value and for decreasing its value if the d.c. voltage goes below the desired value.
These variations can arise from changes in the load for the auxiliary rectifier, changes in the d.c. voltage provided by the power source or from changes in the width of the pulses applied to the primary winding as a result of the regulation of the main rectifying means.
In such a system the series regulators must be kept in the linear mode, i.e. they must always have at least a minimum impedance so that they can perform the regulatory function. Even if the minimum impedance could theoretically be reasonably small, it is generally considerably larger so as to ensure that regulation can occur under extreme conditions. Inasmuch as the impedance is generally resistive, the efficiency of the supply is reduced because of power dissipation occurring therein.
A particularly efficient type of series regulator is described in the proceedings of Powercon 9 for 1982 which are under a registered copyright of Power Concepts Inc. The article by Steven H. Pepper is at D-3, page 1 and is entitled "A New High Efficiency Post-Regulation Technique For Multiple Output Converters." This regulator is comprised of a current transformer having its low turn windings respectively connected in series with the load during alternating half cycles. Its secondary winding is coupled to a first diagonal of a rectifying bridge and the collector emitter path of a transistor is connected across the second diagonal. Direct current voltage at the output is compared with a reference voltage, and the results of the comparison are used to control the base electrode of the transistor. It acts as a variable clamp that effectively inserts a variable load across the second diagonal for different portions of cycles of the a.c. wave from the power source. The variation in load that is reflected to the primary is much smaller in size because of the high turns ratio between the secondary and primary windings and can be modeled as a voltage opposing that from the power source or as a resistor. The theoretical efficiency of such a regulator is about 98%, but in actual practice it is significantly less.